tests: fix rootdir test for today's Fedora

Something has changed in the Fedora's Kernel (in F39 or F40?) so that
special files do not work correctly below /tmp:

    $ sudo mknod -m 666 /tmp/null c 1 3
    $ cat /tmp/null
    cat: /tmp/null: Permission denied

This has broken our 22-rootdir.tst - the fix for this is to create the
rootdir below "$HOME" (one of the possibilities).

While touching this file, let's separate the --rootdir directories for
both isolation types.

mock/integration-tests/22-rootdir.tst

@@ -7,14 +7,17 @@ fi
 . "${TESTDIR}/functions"
 set -e
 
-TMPDIR=$(mktemp -d)
-cleanup() { rm -rf "$TMPDIR"; }
+# creating special files (e.g. /dev/null doesn't work below tmpfs)
+WORKDIR=$(mktemp -d -p "$HOME")
+
+cleanup() { rm -rf "$WORKDIR"; }
 trap cleanup EXIT
 
 header "mock with --rootdir option"
 
 for isolation in simple nspawn; do
-    runcmd "$MOCKCMD --isolation=$isolation --rootdir=$TMPDIR --scrub=chroot"
-    runcmd "$MOCKCMD --isolation=$isolation --rootdir=$TMPDIR --scrub=bootstrap"
-    runcmd "$MOCKCMD --isolation=$isolation --rootdir=$TMPDIR --bootstrap-chroot ${TESTDIR}/test-C-1.1-0.src.rpm"
+    rootdir=$WORKDIR/$isolation
+    runcmd "$MOCKCMD --isolation=$isolation --rootdir=$rootdir --scrub=chroot"
+    runcmd "$MOCKCMD --isolation=$isolation --rootdir=$rootdir --scrub=bootstrap"
+    runcmd "$MOCKCMD --isolation=$isolation --rootdir=$rootdir --bootstrap-chroot ${TESTDIR}/test-C-1.1-0.src.rpm"
 done